Generation of a drive signal for sound transducer
An apparatus for generating a drive signal for a sound transducer ( 109 ) comprises a sound generator ( 101 ) which provides an input audio signal. A divider ( 101 ) divides the input audio signal into at least a low frequency signal and a high frequency signal and an expander ( 105 ) generates an expanded signal by applying a dynamic range expansion to the low frequency signal. A combiner ( 107 ) then generates the drive signal by combining the expanded signal and the higher frequency signal. The threshold for applying the dynamic range extension may be adjusted depending on the amplitude of the low frequency signal. The low frequency signal may furthermore be compressed into a narrow frequency band around a resonance frequency. The approach may allow improved audio quality especially from high Q low frequency sound transducers by attenuating decay parts of bass signals thereby reducing sustain or ringing for bass notes.
1. An apparatus for generating a drive signal for a sound transducer, the apparatus comprising:
a source for providing an input audio signal;
a divider for dividing the input audio signal into at least a low frequency signal and a high frequency signal;
an expander for generating an expanded signal by applying a dynamic range expansion to the low frequency signal, the expander further for adapting an application of the dynamic range expansion to the low frequency signal in response to a dynamically varied threshold determined as a function of at least an averaged amplitude level indication for the low frequency signal; and
a combiner for generating the drive signal by combining the expanded signal and the higher frequency signal.
2. The apparatus of claim 1 wherein the expander is arranged to attenuate the low frequency signal if the input audio signal meets a first criterion.
3. The apparatus of claim 2 wherein the first criterion comprises a requirement that an amplitude level of the low frequency signal is below a threshold.
4. The apparatus of claim 2 wherein the expander is arranged to delay an application of a full attenuation of the low frequency signal following the detection of the first criterion being met.
5. The apparatus of claim 2 wherein the expander is arranged to terminate applying attenuation to the low frequency signal in response to a detection that the input audio signal meets a second criterion; and to delay the termination of applying attenuation to the low frequency signal following the detection of the second criterion being met.
6. The apparatus of claim 1 further comprising:
an amplitude averager for determining the averaged amplitude level indication for the low frequency signal; and
a multiplier for setting a characteristic of the dynamic range expansion in response to the averaged amplitude level indication.
7. The apparatus of claim 6 wherein the characteristic is a criterion for applying an attenuation to the low frequency signal.
8. The apparatus of claim 7 wherein the criterion comprises a requirement that a current amplitude is below an amplitude threshold, and the multiplier is arranged to determine the amplitude threshold in response to the averaged amplitude level indication.
9. The apparatus of claim 8 wherein the multiplier is arranged to determine the amplitude threshold substantially as:
T=c·A A
where T is the amplitude threshold, c is a constant and A A is an averaged amplitude level of the low frequency signal indicated by the averaged amplitude level indication.
10. The apparatus of claim 6 wherein a time constant for determining the averaged amplitude level indication is between 75 and 200 msec.
11. An apparatus for generating a drive signal for a sound transducer, the apparatus comprising:
a source for providing an input audio signal;
a divider for dividing the input audio signal into at least a low frequency signal and a high frequency signal;
an expander for generating an expanded signal by applying a dynamic range expansion to the low frequency signal;
a combiner for generating the drive signal by combining the expanded signal and the higher frequency signal; and
frequency compressor arranged to perform a frequency compression of at least one of the expanded signal and the low frequency signal from a first frequency interval to a smaller second frequency interval corresponding to a resonance frequency of the sound transducer.
12. The apparatus of claim 11 wherein the frequency compressor is arranged to perform the frequency compression of the low frequency signal prior to the dynamic range expansion, the apparatus further comprising:
an amplitude average for determining an averaged amplitude level indication for the low frequency signal component prior to the frequency compression; and
a multiplier for setting a characteristic of the dynamic range expansion in response to the averaged amplitude level indication.
13. The apparatus of claim 11 wherein the frequency compressor comprises:
an amplitude detector for generating an amplitude signal for the at least one of the low frequency signal and the expanded signal;
a frequency generator for generating a carrier signal in the second frequency interval; and
a modulator for generating a frequency compressed version of the at least one of the low frequency signal and the expanded signal by modulating the carrier signal by the amplitude signal.
14. The apparatus of claim 13 wherein the expander is further arranged to determine whether to apply the dynamic range expansion in response to the amplitude signal.
15. A method of generating a drive signal for a sound transducer, the method comprising:
providing an input audio signal;
dividing the input audio signal into at least a low frequency signal and a high frequency signal;
generating an expanded signal by applying a dynamic range expansion to the low frequency signal and adapting an application of the dynamic range expansion to the low frequency signal in response to a dynamically varied threshold determined as a function of at least an averaged amplitude level indication for the low frequency signal; and
generating the drive signal by combining the expanded signal and the higher frequency signal.